(1) Field of the Invention
The invention described herein relates to electronic circuits and their use in shaping power envelopes, and sonar systems making use of a shaped power envelope.
(2) Description of the Prior Art
It is known in the art to use pulsed (or gated) waveform in active sonar systems. These pulsed sonar systems may use signals that are continuous wave (CW), linear frequency modulated (LFM), stepped FM, etc. Often, two power levels are used: high power for long-range targets and low power for short-range targets. These predetermined power levels are used as the supply for the transmitter amplifier, which amplifies the gated signal and applies it to the transducer. Gating the transmit signal is equivalent to transmitting a signal with a square pulse envelope.
There are certain advantages, however, to transmitting a signal with a nonsquare-shaped envelope. For example, in the case of a gated sinusoid, the bandwidth of the transmitted signal is actually the bandwidth of the square pulse (a sinc function, in the frequency domain) centered about the frequency of the sinusoid. This gating results in sidelobes in the spectrum that are 13 dB down from the mainlobe. To reduce the sidelobes, it is necessary to shape the envelope of the transmitted sinusoid. In fact, sidelobes can be practically eliminated if a Gaussian-shaped envelope is used, but only at the expense of a wider mainlobe. Nevertheless, this condition may be desirable in certain applications.
When using a nontrivial shape for the envelope of a transmitted signal, signal generation becomes a more complex process than simple gating. One approach uses a low-level digital or analog electronics to generate the desired signal and its envelope, which is then applied to a linear power amplifier for application to the transducer. However, linear amplifiers are large, expensive, and inefficient. This inefficiency results in the internal dissipation of a considerable amount of power, which leads to massive heat transfer issues.
A need exists for a device which can efficiently provide control of a power envelope. Rapid switching, as well as low power loss, is required in order to generate a shaped power envelope efficiently.